• Apache. The Apache web server is by far the most widely used web server on the internet today. Since April 1996, Netcraft noted that Apache has overtaken Microsoft's IIS web server as the most popular web server for secure content and, as of March 2007, they calculate that it accounts for 60% of all domains, making it more popular than all other web servers combined. 
  Apache is built on a modular design, which allows for extra functionality to be easily integrated into the server. Configuration is powerful and allows for tremendous flexibility and control in how the server functions. As with many Open Source projects, it can be built to run on a diversity of systems (UNIX, Linux, Windows, Mac-OS, see LAMP).

  Market share for top servers across all domains, August 1995--March 2007 (Source)

• GMT. Generic Mapping Tools are a collection of tools to produce high quality world maps. The tools come with a collection of high resolution data for coastlines, rivers, boundaries, and geographic features. It can easily be integrated with other geographical data such digital terrain elevation data and satellite imagery. The project was started by Paul Wessel at the University of Hawaii in 1988, and is now used by thousands of scientists and engineers worldwide. Wikipedia use GMT for many of their maps.
• R. R is a software environment for statistical
  computing and graphics, although sometimes it is described as a statistical programming language. Also known as GNU S, it is broadly compatible with the S programming language developed by Bell Labs and has a rich set of modular add-on packages, many of which are contributed by the R community and available at the Comprehensive R Archive Network. R is notable for its ability to produce publication-quality graphical output, as well as its extensibility and flexibility.
• SugarCRM. Based on the LAMP stack, sugarCRM is a customisable customer relationship management package available as both an Open Source download as well as professional and enterprise versions with extended functionality and support. Sugar features sales, marketing and customer support management among other features. According to Sheryl Kingstone of the Yankee Group, "for a business model that owns, at best, less than one percent of the CRM market, they are getting a lot of buzz". The SugarCRM public license is not currently recognised by the Open Source Initiative.
  
• MySQL. MySQL is a relational database management system that, according to its owning company, has over 10 million installations worldwide. It is extensively used for database backended web applications and websites (see also LAMP) due to its simplicity of use. Some have suggested that MySQL is not a serious database because of the lack of certain features, however, with the latest release, MySQL offers much of the functionality of heavyweight database products.
  
• Perl. Recognised for its power in text processing, Perl is a dynamic programming language that is used in a diversity of situations. Perl was the key way to serve dynamic pages over the CGI interface in the late 1990s. It has tremendous user community support with a an estimated one million users worldwide and there is a broad depth of additional functionality provided by the Comprehensive Perl Archive Network. Perl is often one of the key components in the LAMP stack, and is sometimes described as the "duct tape of the internet" see also LAMP).
  
• PHP. PHP is a scripting language designed to work primarily on the web but it is also capable of being used as a standalone application. According to the project's home page, PHP now runs on approximately 20 million domains, which makes it probably the most popular web scripting language: many web applications, such as Drupal and Joomla , and much of Yahoo is powered by PHP. In a fascinating interview, the creator of PHP Rasmus Lerdorf discusses its background and mentions a number of cool projects. One striking example is Sahana, an Open Source disaster management tool developed in the aftermath of the 2004 Asian tsunami. It has since been deployed in helping to save lives during the Pakistan earthquake in 2005, the mudslide disaster in the Phillipines in 2006, and the Jogjarkata earthquake in Indonesia in 2006 (see also LAMP).
  
• Java. Developed by Sun Microsystems in the 1990s, Java is an object-orientated programming language. It was designed with the goal of being able to be run on a wide range of computer hardware to simplify cross-platform development. In November 2006, Sun announced that Java would be released as Open Source Software. As well as being widely used on the web, Java has been used to develop other major applications.
  
• GNU. The GNU project is probably the most influential of all Open Source projects , without which Open Source as we know it today would not have happened. The project was started in 1984 by Richard Stallman with the goal of producing a completely free UNIX-like operating system. GNU stands for "GNU's Not UNIX", a self-referential acronym that reflects the humour and word play often found amongst free software developers. GNU itself has generated a number of significant projects, including:

• gcc: the GNU compiler collection, the standard compiler for free UNIX-like operating systems and MacOS. Originally released as a C compiler, it also supports C++, Fortran, Pascal, Java, Ada and more. It has been used to build Linux, BSD, Mac OS and Nextstep. Because of its flexibility in building for diverse platforms, it is the most common compiler when developing for a range of applications
• GNOME: an easy to use graphical desktop environment and development system
• The Gimp: an image manipulation program.

To date, the GNU foundation is responsible for over 5000 packages.

• Open Office. Developed by Sun Microsystems and similar in functionality to Microsoft Office, Open Office is a free office suite available for a variety of operating systems including Linux, Solaris, Microsoft Windows and Mac OS X. The mission statement by OpenOffice.org is "to create, as a community, the leading international office suite that
  will run on all major platforms and provide access to all functionality
  and data through open-component based APIs and an XML-based file format".
• Drupal and Joomla. Written in PHP with MySQL database backends, both are two content management systems (CMS) for building websites. Both CMSs are often offered as a standard package by hosting providers and both have a number of websites running from their products. As is common with many Open Source projects, each has their own advocates and detractors; however the rivalry often has mutual respect and drives each product to improve.
• LAMP. LAMP is an acronym for the combination of Linux, Apache, MySQL and PHP (often Perl or Python) that work together to enable dynamic websites and web servers. They are all commonly available on most Linux distributions and as a consequence, although they have not been specifically designed to work with each other, the Open Source community has readily applied them to the web, resulting in the most common way to build a dynamic web server.
  
• Wikipedia. Describe themselves as "a multilingual, web-based, free content encyclopedia project", Wikipedia have taken people's imaginations by storm due to the rapidity of their recent popularity. Created by volunteers, it has often been criticised for inaccuracies but, as is often in Open Source communities, the contributors have worked together to address these issues. In some respects, Wikipedia is beginning to resemble Hitchhiker's Guide to the Galaxy by Douglas Adams. Wikipedia itself runs on the Open Source wiki - mediawiki.